Aim: To examine whether a longer time to pregnancy is associated with preterm births distinguished as either spontaneous or indicated delivery.

Methods: A cross-sectional survey was conducted targeting parous women aged 20-44 years old residing across Japan whose first-born children were singletons. Information on reproductive history including time to pregnancy (TTP) and use of assisted reproductive technology (ART) was collected using internet-based questionnaire. A delivery was judged spontaneous if the mother had either labor and/or membrane rupture at hospital admission. Multinomial logistic regression analyses were performed to estimate odds ratios and 95% confidence intervals (CI) for spontaneous and indicated preterm birth at ＜ 34 or at ＜ 37 weeks of gestation while adjusting for potential confounders.

Results: The analytic sample was n=4,208 mothers with first-born singletons. After adjusting for age, use of ART, and other possible confounders, women with TTP of 12＋ months had significantly higher odds of spontaneous preterm birth at ＜ 34 weeks (OR 4.55, 95% CI 1.10-18.77) but not at ＜ 37 weeks (OR 1.07, 95% CI 0.65-1.75), compared to those with TTP of ＜ 6 months. Women with unknown TTP tended to have higher odds of spontaneous preterm birth at ＜ 34 weeks (OR 3.67, 95% CI 1.02-13.19) and at ＜ 37 weeks (OR 1.38, 95% CI 0.98-1.96), though not statistically significant. There was no significant association with TTP and indicated preterm birth.

Conclusions: Compared to mothers with shorter TTP, those with longer TTP are more likely to experience spontaneous preterm birth.